Remote Sounding of Cloud Compositions from NOAA IV and Nimbus VI Infrared Sounders.

Abstract

Abstract : A spectral infrared radiative transfer model based on the discrete-ordinate method has been developed for atmospheres containing clouds and absorbing gases above, below and within the cloud. The transfer model takes into consideration the inhomogeneity of the cloudy atmosphere, the gaseous absorption in scattering cloud layers and the wavenumber dependence of radiative transfer. In addition, the cloud layer is further divided into a number of sub-layers to account for the non-isothermal and inhomogeneous cloud characteristics. The spectral infrared transfer program is applied to VTPR channels of the NOAA 4 satellite to simulate upward radiances in cirrus cloud conditions. Single-scattering properties for ice crystals are calculated assuming ice cylinders 200 and 60 micrometers in length and width, respectively, randomly oriented in a horizontal plane. Comparisons between satellite observed and theoretically simulated upward radiances are carried out for selected cirrus cloud cases. Incorporating atmospheric profiles obtained from radiosonde and the observed cloud information into the spectral transfer program, we show a systematic agreement between observed and computed upward radiances. Systematic reduction patterns of the upward radiance caused by the increase of the cloud ice content are demonstrated for VTPR channels employing tropical and midlatitude atmospheric profiles. Having the quantitative relationships between upward radiance and ice contents, procedures are described for the inference of the cloud ice content and cloud amount. The proposed method has been successfully applied to the three cirrus cloud cases.